1. Field of the Invention
The present invention relates to a process of joining metal members, such as connection terminals between circuit boards, typically between flexible and rigid circuit boards.
The term "circuit board" as referred to herein means a structure composed of an insulating base board and an electric circuit formed on the base board. The term "flexible circuit board" means a circuit board having a flexible base board.
2. Description of the Related Art
Processes of joining connection terminals between circuit boards are disclosed in Japanese Unexamined Patent Publication (Kokai) No. 51-20589, Japanese Unexamined Patent Publication (Kokai) No. 51-122773, Japanese Unexamined Patent Publication (Kokai) No. 53-83077, and Japanese Unexamined Patent Publication (Kokai) No. 4-186697.
Japanese Unexamined Patent Publication (Kokai) No. 51-20589 disclosed a process of joining connection terminals between circuit boards by interposing an electroconductive resilient material between the electrodes of two circuit boards and holding these circuit boards so that the electroconductive resilient piece is kept compressed between the electrodes. This process cannot provide a reliable joint at low or high temperatures, because the electroconductive resilient piece is a metal contact, an electroconductive rubber piece, or an electroconductive coil spring.
Japanese Unexamined Patent Publication (Kokai) No. 51-22773 disclosed a process, in which connection terminals of a flexible circuit board are joined to connection terminals of an electronic component by a cushion kept compressed. This process also has the same problem as that encountered in the above-mentioned process, i.e., it cannot provide a reliable joint at low or high temperatures.
Japanese Unexamined Patent Publication (Kokai) No. 53-83077 disclosed a process of joining connection terminals of a flexible circuit board to connection terminals of a rigid circuit board by thermocompression bonding, in which an excessive amount of molten solder is allowed to flow away through the holes opened at sites corresponding to connection terminals of the flexible circuit board. This process involves the use of a flux which cannot completely be removed from the joint because of its structure and a flux residue reduces the reliability of the joint.
Japanese Unexamined Patent Publication (Kokai) No. 4-186697 disclosed a process, in which a thermoplastic resin is interposed between a connection terminal of a flexible printed circuit board and a connection terminal of another circuit board, and an ultrasonic vibration is then used to effect solid phase bonding. This process has a problem that plural bonding points form an uneven surface which is unfavorable for providing a uniform bonding.
There is also known a process of joining connection terminals by forming a solder bead thereon and applying ultrasonic vibration to suppress generation of an oxide film on the solder bead, so as not to remove an oxide film. This process is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 62-172676, in which a solder bead is provided only on one counterpart of the pair of connection terminals to be joined together and the other counterpart is not provided with a solder bead.
When the solder bead of one connection terminal is abutted against the other connection terminal having no solder bead and ultrasonic vibration is then applied to the abutted assembly, the ultrasonic vibration is only effective to remove an oxide film from the solder bead of the former connection terminal but is not effective to remove an oxide film from the latter connection terminal, because the solder bead material is softer than the connection terminal material.
Therefore, a flux is necessary to remove oxide films both from the solder bead (on the former connection terminal) and from the latter connection terminal.
Japanese Unexamined Patent Publication (Kokai) No. 2-12930 disclosed another process, in which a solder material is merely placed between the connection terminals to be joined together. In this process, the solder material is abutted against the connection terminal, so that an ultrasonic vibration cannot remove oxide films from both the solder material and the connection terminal, and therefore, a flux is necessary to remove the oxide films.